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Wednesday, October 2, 2013

The Unbearable Baldness of Black Holes

One of the most awe-inspiring properties of black holes is their absolute simplicity, or as John Wheeler famously put it, "black holes have no hair". As their progenitor collapses, its memory is forever lost, and all that remains is a quiescient, bald black hole. In a new article in Physical Review Letters, a team of scientists (that only by chance includes me...) has shown that black holes can nevertheless "grow hair" in the presence of matter, connecting them to the rest of the host galaxy.
Black holes are almost xeroxed copies of one another, differing at most in mass and rotation. These objects are described by a solution discovered by Roy Kerr in 1963. Remarkably, Kerr black holes are ubiquitous in almost any other theory of gravity, to the extend that the "Kerr hypothesis" is the current paradigm in astrophysics.

First time I saw this picture was in one of Stephen Hawking's popular-science books, probably 'Brief History of Time'. It is supposed to describe the 'baldness' that this post refers too, am I the only one finding it a bit pathetic? :)

We have shown that in simple and attractive extensions of Einstein's theory (known as scalar-tensor gravity) black holes may not be described by the Kerr metric, as was previously thought. The crucial ingredient is the matter surrounding astrophysical black holes, typically in the form of accretion disks. The presence of matter triggers an instability that forces the bald Kerr black hole to develop a new charge -- a "scalar hair" -- connecting it to the matter around it and possibly to the entire galaxy.
This hair growth is accompanied by a peculiar emission of gravitational waves, potentially by upcoming laser interferometers, which may test the Kerr hypothesis and probe the very foundations of gravity.